Detergent organisation in crystals of monomeric outer membrane phospholipase A

The structure of the detergent in crystals of outer membrane phospholipase A (OMPLA) has been determined using neutron diffraction contrast variation. Large crystals were soaked in stabilising solutions, each containing a different H2O=D2O contrast. From the neutron diffraction at five contrasts, the 12 A resolution structure of the detergent micelle around the protein molecule was determined. The hydrophobic b-barrel surfaces of the protein molecules are covered by rings of detergent. These detergent belts are fused to neighbouring detergent rings forming a continuous three-dimensional network throughout the crystal. The thickness of the detergent layer around the protein varies from 7–20 A. The enzyme s active site is positioned just outside the hydrophobic detergent zone and is thus in a proper location to catalyse the hydrolysis of phospholipids in a natural membrane. Although the dimerisation face of OMPLA is covered with detergent, the detergent density is weak near the exposed polar patch, suggesting that burying this patch in the enzyme s dimer interface may be energetically favourable. Furthermore, these results indicate a crucial role for detergent coalescence during crystal formation and contribute to the understanding of membrane protein crystallisation. 2002 Elsevier Science (USA). All rights reserved.